In the production of semiconductor devices, the lithography technique for forming integrated circuit patterns on semiconductor wafers has strikingly been advanced. In particular, the wavelength of the light rays used in the lithography has increasingly been reduced and changed, for instance, from i-rays emitted from mercury lumps (365 nm) to light rays emitted from excimer lasers such as KrF excimer laser (248 nm) as the degree of integration of LSI's increases. Such reduction in the wavelength of light rays used in the lithography requires the use of silica glass optical parts having high transmittance to ultraviolet rays as optical parts incorporated into the optical system used in the lithography instead of the conventional multicomponent glass optical parts, in particular, the use of chemically purified synthetic silica glass optical parts having highly improved transmittance to ultraviolet rays. Moreover, such optical parts incorporated into these optical systems of high precision should satisfy more strict requirements such as quite excellent transmittance to ultraviolet rays and very high optical homogeneity.
In general, the glass sometimes shows abrupt change in its refractive index appearing in the streak-like or lamellar configuration. Such a heterogeneous structure is generally called "cord" and is distinguished from the homogeneity in the refractive index. In addition, the glass has heterogeneous structures which are accompanied by a change in the refractive index of the glass and such heterogeneous structures are caused due to the heterogeneity of compositions and the heterogeneity in thermal and other conditions during the production thereof. In the optical glass, such a heterogeneous structure including such a cord is a defect which should be removed since the structure would distort the optical path. Furthermore, the optical glass must also satisfy the strict requirements for the homogeneity in the refractive index to an extent depending on the precision required for specific optical systems in order to obtain a highly precise transmitting wave surface.
In the production of optical glass for general use, the homogenization of the refractive index is carried out by compulsorily stirring a melt of molten glass and then gradually cooling the melt under quite strict temperature control in order to satisfy these requirements.
However, it is difficult to heat the silica glass in a crucible up to a high temperature at which the silica glass is in a molten state having a low viscosity because of the nature peculiar thereto. On the contrary, the silica glass has a sufficiently high viscosity even at a high temperature and is insufficient in the flyability. For this reason, it is in fact difficult to obtain silica glass having satisfactory optical homogeneity through a mixing operation such as stirring.
As means for homogenizing such silica glass, for instance, U.S. Pat. No. 2,904,713 discloses a method for homogenizing the whole composition of silica glass which comprises locally heating the silica glass while supporting both ends thereof by a lathe for processing silica glass to partially form a molten zone within the silica glass, twisting, expanding and contracting the molten zone through application of an external force thereto so that the ends approach or are separated from one another while rotating the ends in different manners to move the molten zone over the entire length of the silica glass.
U.S. Pat. No. 3,485,613 discloses a method for performing homogenization of silica glass, called lateral zone melting method, which comprises setting the viscosity of the molten portion of the silica glass, i.e., the molten zone at a level of not more than 1013 P and moving the molten zone over the entire length of the silica glass while twisting the molten zone of the silica glass by rotating both ends of the silica glass in the same direction and at different numbers of revolutions or in different directions.
These techniques for homogenizing silica glass do not intend to eliminate the cord present in silica glass, but U.S. Pat. No. 5,086,352 and Japanese Un-examined Patent Publication (hereunder referred to as "J.P. KOKAI") No. Hei 3-88742 disclose that the cord of silica glass can be removed through these homogenization techniques for silica glass.
Moreover, DEOS No. 42 04 406 A1 discloses a method for eliminating cords of natural and synthetic silica glass in the three directions perpendicular to one another, in which the homogenization of the silica glass while establishing a second rotation axis perpendicular to a first rotation axis for homogenization is carried out by molding a silica glass formed article subjected to a first homogenization in, for instance, an inverted T-shaped mold of graphite having a square cross section to form a rod-like synthetic silica glass having a square cross section and a longitudinal axis in the direction perpendicular to the rotation axis, for the first homogenization treatment, of the rod-like silica glass formed article subjected to the first homogenization, carrying out a second homogenization while using the longitudinal axis as a second rotation axis to completely eliminate cords in the three directions perpendicular to one another in quite high efficiency.
In addition, the method for homogenizing a silica glass material along the two axes perpendicular to one another and for removing cords as disclosed in the foregoing DEOS No. 42 04 406 A1 permits, for the first time, the economical production of a highly homogeneous silica glass formed article free of cord, which may be used in, for instance, the photolithography.
Incidentally, the synthetic silica glass has been used as a silica glass raw material for optical use because of high purity, in particular, a low content of metallic impurities and excellent light transmittance to light rays falling within the ultraviolet region and is prepared according to the direct method or the soot method. For instance, the vapor phase axial deposition method (VAD method) basically comprises the steps of subjecting a volatile silicon compound to the flame hydrolysis, depositing, as a layer, silica fine particles formed during the flame hydrolysis on a substrate which is rotated and then transparentizing the deposit of silica particles. A lamellar heterogeneous structure is thus formed during growing a synthetic silica glass ingot through the transparentizing step and this serves as a lamellar heterogeneous portion in the resulting synthetic silica glass ingot.
Such a lamellar growing stripe in the synthetic silica glass ingot is formed in the form corresponding to the shape of the growing front of the synthetic silica glass and the stripe accordingly form a cord having a curved surface such as approximately a hemispherical surface. A cord having such a curved surface is likewise observed in the synthetic silica glass ingot produced by the direct method and such a cord is considered to be one of the cords present in the synthetic silica glass whose removal is most difficult.
Therefore, if the synthetic silica glass which is once homogenized by, for instance, a simple lateral zone melting method is examined by a strain detector and an interferometer, it would be confirmed that the optical homogeneity thereof in the direction perpendicular to the rotation axis used during the homogenization is lower than that observed in the direction along the rotation axis. This clearly indicates that the effect of the mixing is almost ineffective for the homogeneity in the direction parallel to the rotation axis. Accordingly, it is concluded that the simple lateral zone melting method permits the removal of unidirectional cord and homogenization, but it is very difficult to achieve the removal of cord and homogenization in the three directions perpendicular to one another and that the method is less efficient.
In order to achieve the removal of cord and homogenization in the three directions through homogenization treatment according to such a simple lateral zone melting method, the lateral zone melting procedures should be repeated over a number of times and this requires much expenses. Moreover, if the lateral zone melting procedures are repeated to remove cord, the optical homogeneity of the resulting product in the direction perpendicular to the rotation axis is not always sufficiently high, the silica glass formed article does not show the quality required for optical use and also suffers from a problem of cost.
As seen from these points, the method for homogenizing silica glass comprising natural and synthetic silica glass as disclosed in the foregoing DEOS No. 42 04 406 A1 is an excellent method and permits the production of silica glass free of cords and having high homogeneity in the three directions perpendicular to one another. However, the silica glass formed article as a product prepared by this method shows low transmittance to, in particular, ultraviolet rays falling within the wavelength range of not more than 250 nm and emits fluorescent light rays through irradiation with ultraviolet rays. Therefore, it has been found that the method is insufficient for forming a silica glass article capable of transmitting light emitted from an excimer laser or ultraviolet rays falling within the same wavelength range.
It would be assumed that this problem is caused due to the following fact. The rotation axis for the second homogenization is established by molding synthetic silica glass in a graphite mold according to a high temperature heat-molding method to give a synthetic silica glass molded article, the resulting silica glass article is contaminated by graphite through the contact between the glass and the graphite mold at a high temperature during the molding, and thus the impurities originated from the graphite and present in the silica glass molded article are uniformly distributed throughout the silica glass article during the second homogenization step.
Metallic impurities should be removed from the silica glass as much as possible to produce silica glass articles having good optical stability to the light rays emitted from an excimer laser. For instance, the optical parts of synthetic silica glass must have alkali and alkaline earth metal contents each of which is not more than 50 ppb and transition metal contents each of which is not more than 10 ppb.
In general, the synthetic silica glass formed articles for optical use have been formed according to a high temperature heat-molding method which makes use of a graphite mold. However, when forming a high purity silica glass article capable of transmitting light emitted from an excimer laser or ultraviolet rays falling within the same wavelength range, the formed article is inevitably contaminated through the contact with the graphite mold. For this reason, the application of the high temperature heat-molding method using a graphite mold is limited to the forming steps subsequent to the final homogenization step. If the high temperature heat-molding method using a graphite mold is simply applied to the forming steps subsequent to the final homogenization step, the silica glass formed article is contaminated at only the portions which comes in contact with the graphite mold and accordingly, any influence of such impurities can be eliminated by, if necessary, scraping such contaminated portions. In this case, however, the graphite mold has been produced from graphite having a purity as high as possible to minimize the contamination due to contact between the silica glass and the graphite mold.
Accordingly, the principal object of the present invention is to solve the problem associated with the method for preparing a highly homogeneous silica glass formed article free of cords in the three directions perpendicular to one another, which relates to contamination of the article through the contact with the graphite mold, as disclosed in the foregoing DEOS No. 42 04 406 A1 and to further improve the cord-removing effect and the homogeneity of the resulting formed article.